The girl I like is shy and quiet, but every my approach to her has failed, at least how it looked to me.
Today I asked her out and she refused, motivating she's into some volunteer business on weekends (and that's true, as it turned out to be).
What would you recommend to do, Secret Admirer?

get another one. dont u understand u have no chance?

oh dear,

This forum is dead. Used to be alive somewhat years ago.

Do you flirt with her? How does she reacts?

Try to challenge her to do things for you. Your changes of being liked would actually be increasing.